Leftovers save both time and money — why cook twice when once will do? But if you’re making any of these microwave mistakes, you could pay a different sort of price.

1. Microwaving in plastic containers

You no doubt know not to use metal inside a microwave, as it can spark. But plastic is also problematic. Some tests have suggested that bisphenol-A (BPA), a chemical used to harden plastic — and one that has been linked to health problems including diabetes, infertility and cardiovascular disease — leaches from plastic containers during microwave heating. 

Don’t take too much comfort from the words ”microwave safe” on a plastic container. “The FDA does not have a definition for the term ‘microwave safe,’” wrote Food and Drug Administration Spokesperson Juli Putnam in an email to SafeBee. The Consumer Product Safety Commission also told SafeBee it has no definition for “microwave-safe.” 

Related: Plastic: A Bunch of Seriously Good Reasons to Just Say No

If you are going to use plastic in the microwave, don’t use worn or cracked containers. Over time, the heat of a microwave can break down lightweight polycarbonate plastic, according to the National Institute of Environmental Health Sciences. This may cause them to release more BPA.

Never use margarine tubs or other cold food storage containers. “These are designed for refrigerator or freezer storage of foods and are intended for those uses only,” cautions the United States Department of Agriculture (USDA). “They are not heat stable and chemicals from the plastic may migrate into the food during heating.” 

Stick with glass and ceramics for microwave heating. With melting temperatures well above 1,000 degrees Fahrenheit, they’re the most stable non-metal choices in our kitchens.

2. Covering the dish with plastic wrap

Covering a dish before reheating in the microwave is smart: It helps prevent splatters, keeps food moist and also helps heat food more evenly. Covering a dish with plastic wrap, though, is less smart. Heating food in a container covered with plastic can create chemical gases that migrate to the food — even when the plastic isn’t touching the food directly, according to a USDA consumer fact sheet.

A glass or ceramic plate makes a good lid. Or consider using a paper towel. Just be sure to use white paper towels, and not ones with colored designs, advises the maker of Bounty and Viva paper towels.

3. Defrosting on foam tray packaging

The foam trays on which meat and poultry are often sold are inappropriate for microwave cooking or defrosting. Per the USDA, “foam insulated trays and plastic wraps on fresh meats in grocery stores are not intended by the manufacturer to be heated and may melt when in contact with hot foods, allowing chemical migration into the food.” 

Related: How to Thaw Meat

4. Opening the lid toward your face

The most common injury from microwave use is a burn from a spill, but steam can burn you, too. So take care when removing the lid. Use a hot pad to protect your hand and angle the lid away from your body as you remove it.

5. Failing to stir halfway through

To kill any harmful bacteria, the USDA recommends heating leftovers to an internal temperature of 165 degrees as measured with a food thermometer. Since microwaves cook unevenly, stop the microwave during the cook time to flip over or stir foods. This way you’re less likely to end up with cold spots where bacteria might linger.

Related: Quiz: Toss It or Eat It? 

6. Eating the food right away

Have you ever noticed that some frozen foods meant for the microwave instruct you to let the food stand for a couple of minutes after cooking? There’s a reason. According to foodsafety.gov, “letting your microwaved food sit for a few minutes actually helps your food cook more completely by allowing colder areas of food time to absorb heat from hotter areas of food. That extra minute or two could mean the difference between a delicious meal and food poisoning.”

What containers and lids do you use when reheating food in the microwave? Let us know by leaving a comment below.

Sarah Pinneo is the author of "Julia's Child" and "The Ski House Cookbook." She lives and works in Hanover, NH.